In recent years, there have been increasing demands for devices with higher densities and higher integration degrees in the fields of various electronic devices that require fine processing, such as semiconductor devices. To satisfy those demands, formation of finer patterns is essential. In procedures for manufacturing such semiconductor devices, the photolithography technology plays an important role in the formation of fine patterns.
To increase photolithography resolution, it is necessary to shorten the wavelength of the light source to be used for exposures or increase the numerical aperture of the projector lens. When the numerical aperture is increased, the resolution becomes higher, but the focal depth becomes smaller. As a result, predetermined resolution cannot be achieved with respect to an exposure of a surface having concavities and convexities with a depth equal to or greater than the focal depth. Therefore, the flatness of the substrate needs to be increased. To form even finer patterns, the wavelength λ of each light source is required to have a shorter wavelength. The wavelength λ of each light source used for exposures has been shortened from g-ray (436 nm) to i-ray (365 nm). At present, excimer lasers (248 nm, 193 nm) are being mainly used as the light sources. By the photolithography technology, however, the diffraction limit of a light source is the resolution limit. Therefore, even with the use of a 193-nm ArF excimer laser immersion exposure technique, it is difficult to form patterns of 10 nm or smaller in linewidth.
To form even finer patterns, it is necessary to use the X-ray lithography technology or the electron beam lithography technology. By the X-ray lithography technology, the resolution can be made ten or more times as high as the resolution achieved when an exposure is performed with the use of an excimer laser. By the X-ray lithography technology, however, it is difficult to form a mask, and the device costs are high.
By the electron beam lithography technology, formation of patterns on the order of nanometers can be controlled with high precision, and a greater focal depth than that of an optical system can be achieved. The electron beam lithography has the advantage that a pattern can be drawn directly on a wafer without a mask. However, the electron beam lithography has low throughput, and is costly. Therefore, the electron beam lithography is not suited to mass production.
Further, in lithography using an X-ray or an electron beam, it is necessary to develop a resist in accordance with each exposure method, and there are a large number of problems in terms of sensitivity, resolution, etching endurance, and the like.
To solve those problems, there has been a suggested method in which by near-field light leaking from openings with diameters sufficiently smaller than the wavelength of the light to be emitted, a resist is then exposed and developed, to form a fine pattern. This method is characterized by achieving a spatial resolving power on the order of nanometers, regardless of the wavelength of the light source. The method using near-field light is not subjected to a restriction in terms of the optical diffraction limit, and accordingly, is capable of achieving a spatial resolving power that is a third or less of the light source wavelength. Further, with the use of a mercury lamp or a semiconductor laser as the light source, the size of the light source can be reduced. Accordingly, the device can be made smaller in size, and the costs can be lowered.
As one of the lithography techniques using near-field light, there has been a known method by which a near-field exposure mask having a light shielding layer with openings smaller than the light source wavelength is brought into contact with a resist so that the distance in between becomes 100 nm or less, which is a near-field range, and the fine pattern on the mask is transferred to the resist by a collective exposure. In this operation, the contact properties are critical. It is known that a membrane mask can be used as the near-field exposure mask. It is also known that a resin mask can be used as the near-field exposure mask.
As a method of performing an exposure through contact with the use of a membrane mask, there has been a disclosed near-field exposure method by which the thickness of the mask is reduced to such a value that the mask can be elastically deformed, and the mask is elastically deformed by applying a controlled pressure onto the thinned portion so that the exposure mask is brought into contact with the substrate to be exposed. By this method, however, the required manufacturing process consists of a large number of procedures for forming the mask having a thin-film structure, and the thinner portion of the mask might be broken at the time of pressure application or pressure release.
By a method of performing an exposure with the use of a resin mask, on the other hand, the contact between the mask and the light shielding layer is strong. Therefore, the mask might be broken at the time of mask detachment, or the resist might come off the substrate.
When a resist such as a chemically-amplified resist or a photo cation polymerizable resist that exhibits a development contrast through a reaction with a catalyst that is the acid generated by an exposure, the light shielding layer to be the mask is corroded by the generated acid. As a result, the life of the mask might be shortened.
As described above, conventional near-field masks have room for improvement to achieve excellent contact properties with respect to to-be-exposed substrate over a large area, reduce the number of procedures in the manufacturing process, and increase the durability.
There has been a known method by which a near-field exposure is performed by using resonant light with a light wavelength equivalent to the resonant energy of the molecules forming a resist. The near-field exposure using resonant light is performed as follows.
A first substrate having a photoresist layer formed thereon, and a mask having a mask pattern formed on a transparent second substrate are prepared. The mask pattern is then brought into contact with the resist layer. With the mask being in contact with the photoresist layer, an i-ray (365 nm) is emitted onto the back surface of the mask. As a result, near-field light leaks from the openings of the mask pattern by virtue of the i-ray irradiation, and an exposure is performed. The exposed resist portions react to the light.
After the exposure, the mask is detached from the photoresist layer, and the photoresist layer is developed with a developer. As a result, the exposed portions are dissolved, and a pattern is formed.
As another near-field exposure method, there has been a known method by which a near-field exposure is performed by using nonresonant light with a longer wavelength than the wavelength of light equivalent to the resonant energy of the molecules forming a resist. A mask pattern transfer through a near-field exposure using such a nonresonant wavelength is performed as follows. A resist layer is formed on a first substrate, and a mask having a mask pattern with openings formed on a transparent second substrate is prepared. The mask pattern is then brought into contact with the resist layer. With the mask pattern being in contact with the resist layer, nonresonant light having a longer wavelength than the wavelength of light equivalent to the resonant energy of the molecules forming the resist is emitted on to the back surface of the mask.
As a result, near-field light leaks from the openings of the mask pattern by virtue of the nonresonant light irradiation, but the resist layer does not react to the nonresonant light. However, strong electronic polarization occurs at the edge portions of the mask pattern, and near-field light is generated from the nonresonant light. The molecules forming the resist get dissociated through excitation caused by multiple light absorptions by the near field light generated from the nonresonant light (a multistep transition process).
After the exposure, the mask is detached from the resist layer, and the resist layer is developed with a developer. As a result, the portions exposed by the near-field light generated from the nonresonant light are dissolved, and a pattern is formed. The difference from an exposure using resonant light is that a pattern is formed along the edge portions of the mask pattern. Accordingly, a finer pattern can be formed with the use of nonresonant light.
Since the photosensitive wavelength of the photoresist is in a visible range, a glass material is normally used as the transparent second substrate. To increase the efficiency in the exposure process, the size of the transparent second substrate needs to be made larger. In recent years, the sizes of wafers used in semiconductor manufacturing processes are 300 mm in diameter. Since the near-field exposure method using nonresonant light is a contact exposure method, the size of the transparent second substrate needs to be approximately 300 mm in diameter. Since the mask pattern and the resist layer need to be in contact with each other, the transparent second substrate should have low surface roughness and small warpage.
However, where a glass material of approximately 300 mm in diameter is used as the transparent second substrate, it is difficult for the second substrate to have sufficiently low surface roughness and small warpage over a large area. On the other hand, a Si wafer of 300 mm in diameter can have sufficiently low surface roughness and small warpage. However, such a Si wafer cannot be used as the mask, because visible light cannot pass through Si.
Further, at the time of an exposure, light from the light source enters perpendicularly to the first substrate. When the transparent second substrate is made of glass, the reflectivity at the interface between the air and the glass is 4%, and the loss of the incident light energy is small. However, the reflectivity at the interface between the air and Si is as high as 30%, and the exposure time becomes longer. Therefore, the productivity becomes lower in the exposure process.
Further, there has been a demand for higher-density microfabrication of semiconductor packages, interposers, printed circuit boards, and the like, as semiconductors have been made to have smaller sizes, higher densities, and higher speeds. Particularly, in recent years, at the time of formation of a storage media fine structure pattern or formation of a biochip nanostructure, high-density microfabrication is more and more strongly required. As a mass-production means to satisfy such a technical demand, the nanoimprint technology has been studied in recent years.
The nanoimprint technology has been developed by applying a pressing method using a metal mold to the nanoscale technology, and involves a nanoscale mold processing technique for performing molding by pressing a mold with minute concavities and convexities against an object to be processed. By the nanoimprint technology, patterns with a width of several tens of nanometers can be formed. Compared with an equivalent processing technology using an electron beam, the nanoimprint technology has the advantage that a large number of patterns can be molded at very low costs.
In the nanoimprint technology, the use of near-field light has been suggested. Particularly, an ultrafine pattern of 10 nm or smaller can be transferred with high precision by the nanoimprint technology using near-field light. When a Si substrate is processed, light needs to be emitted onto a glass template. However, this irradiation direction lowers the near-field light generation efficiency.
The use of near-field light has also been suggested for template and pattern forming methods based on the nanoimprint technology. However, when a fine pattern is transferred, the contact between the template and the Si substrate needs to be improved. Therefore, there is still room for improvement to develop an optimum pattern forming method.
The present invention has been made in view of these circumstances, and an object thereof is to provide a near-field exposure mask that can secure contact between a mask and a to-be-exposed object over a large area, a resist pattern forming method, and a device manufacturing method.
Another object of the present invention is to provide a near-field exposure method by which the exposure time can be shortened.
Yet another object of the present invention is to provide a pattern forming method by which an ultrafine pattern can be transferred onto a Si substrate with high precision.
Still another object of the present invention is to provide a high-precision near-field optical lithography technique that can realize double patterning with high efficiency.